The enzymes catalyzing the interconversion of proline and pyrroline-5-carboxylate provide a mechanism for the intercompartmental intercellular and interorgan transfer of redox potential. The proline cycle links glucose oxidation through the hexosemonophosphate-pentose pathway to mitochondrial oxidatine phosphorylation. Ornithine and glutamate can serve as additional sources of pyrroline-5-carboxylate.